I once ask the same question but not on here.

Since that time, i've learned the hard way about a few things, and I hope that you won't have to, so i'll pass on what I know to date.
1. Word of mouth either makes or breaks a bee business.
2. In most states there are state apiary inspectors that MUST certify your bees prior to you selling them. (They verify that the bees or colonies they came from are disease FREE.
3. In my opinion ANY seller of bees that isn't willing to allow you to inspect the bees you are buying shouldn't be trusted. (Even if he/she does have a seller's inspection certification)
4. ALWAYS, again ALWAYS request to see the bees in the box, verify that there is a laying queen and her pattern is good if purchasing a NUC.
5. If you purchase a "Package of bees", Prepare to keep a very close eye on them and to requeen at some point in the first season.
Now, in my opinion and many others will agree, Purchase your bees within your geographical area, TRI state area that is. They are more apt to be adapted to the local area better. If you cannot find bees in that area, choose a seller that has the same, or worse weather than you do.
However, the true #1 thing that you should do prior to getting your own bees, is to go and get some experience with someone else that has plenty of hives.